1. Field of the Invention
The present invention relates to an average current controller, an average current control method and a buck converter using the average current controller; and more particularly, to an average current controller, an average current control method and a buck converter using the average current controller capable of overcoming problems that an average current level is changed and a current control becomes difficult in case when an on-time is short.
2. Description of the Related Art
In general, a driving circuit of a display device such as an LED (Light Emitting Diode) mainly uses a linear control method to control a constant current and a PWM (Pulse Width Modulation) method to use a switch. However, the PWM method capable of maximizing the efficiency is preferred to the linear control method having a low efficiency. And also, since the voltage to drive the LED is often higher than that of the LED, the converter in a type of buck is mainly used.
FIG. 1 is a view showing a conventional off-time control type buck converter.
Referring to FIG. 1, if the switch 103 is turned on, the current flows into a load 105, e.g., an LED, an inductor 104 and a switch 103. At this time, the current flowing into the switch is sensed by an Rcs 107 to thereby found its size.
If the value obtained by converting the current of the switch 103 into a voltage by the Rcs 107 is equal to or greater than a peak level, the switch 103 is turned off since the comparator 101 resets an SR latch 102.
If the switch is turned off, the switch 103 is turned on by setting the SR latch 102 again after a predetermined time by using the timer 106 to measure the off-time.
Accordingly, the controller performs a constant off-time operation to be turned off during a predetermined time, and the output current is determined as the Rcs 107 and the peak level uniformly.
In the above off-time control type buck converter, if a power voltage, an inductor, a load voltage and a current ripple are represented as VS, L, VF and ΔI, respectively, the on-time Ton can be described as follows.
      T    on    =                    L                              V            S                    -                      V            F                              ·      Δ        ⁢                  ⁢    I  
And also, the off-time Toff is determined by the load voltage VF and can be represented as follows:
      T    off    =                    L                  V          F                    ·      Δ        ⁢                  ⁢    I  
As described above, since the off-time control type buck converter controls the constant off-time, the off-time Toff is constant, whereby the current ripple ΔI is proportional to the load voltage VF. Accordingly, as shown in FIG. 2, although the peak current is controlled constantly in case when the load voltage VF is different, the average current is changed because the ripple current is different. Particularly, as increasing the ripple as increasing the load voltage VF, the average current is reduced.
Accordingly, in case when the LED is controlled by using the buck converter using the off-time control method, there are problems as follows.
At first, since the load voltage drops of the LEDs have different characteristics for each manufacturer, it is difficult to control the average current.
Secondly, since the load voltage of the LED has a property being inversely proportional to the temperature, the average currents of the LEDs become different at high temperature/low temperature according to this.
Thirdly, since the whole load voltage is changed in case when the number of loads is changed, the average current of the LEDs are changed.
On the other hand, in the Korean Patent Laid-open Publication No. 10-2012-0019642 (AVERAGE CURRENT CONTROLLER), by comparing the time T1 when the current of the switch reaches the average current IAVG to the time T2 to reach the peak current IPEAK at the average current IAVG, if T1>T2, the level of the peak current IPEAK is raised; and, if T1<T2, the level of the peak current IPEAK is lowered. Since T1 and T2 become the same approximately in such control method, the average current becomes IAVG.
However, in the control method of the above conventional average current controller, two comparators are required for sensing to reach the average current IAVG and the peak current IPEAK, there is a problem that the average current level is changed due to the delay generated from a signal processing relationship of two comparators. And also, in case when the on-time T1+T2 are very short, the control of the current becomes very difficult.